A neurotransmitter is a chemical substance that aids in the transmission of an impulse across the synapse. An impulse will cause the release of a neurotransmitter, which is synthesized and stored in terminal bulbs of the axon. The neurotransmitter will diffuse across the synaptic cleft and Initiate an impulse in the postsynaptic nerve. The neurotransmitter reacts with a receptor-site on the postsynaptic nerve initiating an impulse. The neurotransmitter must be removed from the synaptic cleft to stop the impulse.
a. Acetylcholine. Acetylcholine (Ach) is destroyed by acetylcholinesterase (AchE) in the synaptic cleft.
b. Norepinephrine. Norepinephrine (NE) is removed from the synaptic cleft by:
(1) Reabsorption (reuptake) into the terminal knob.
(2) Destroyed by catechol-o-methyl transferase (COMT).
(3) Destroyed by monoamine oxidase (MAO).
(4) Dilution by diffusion out of the junctional cleft. 5-9. THE ALL OR NONE LAW
This law states that if a stimulus is strong enough to cause a nerve impulse, it will cause the entire fiber to depolarize and not just part of it.
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